1. Field of the Invention
The present invention related generally to the fabrication of semiconductor pixel sensor arrays, and more particularly, to a novel process for forming a microlens structure.
2. Discussion of the Prior Art
CMOS image sensors are beginning to replace conventional CCD sensors for applications requiring image pick-up such as digital cameras, cellular phones, PDA (personal digital assistant), personal computing devices (mobiles, laptops and the like). Advantageously, CMOS image sensors are fabricated by applying present CMOS fabricating process for semiconductor devices such as photodiodes or the like, at low costs. Furthermore, CMOS image sensors can be operated by a single power supply so that the power consumption for that can be restrained lower than that of CCD sensors, and further, CMOS logic circuits and like logic processing devices are easily integrated in the sensor chip and therefore the CMOS image sensors can be miniaturized.
The patent literature is replete with references describing image sensor arrays having a microlens structure and aspects of their manufacture. United States Patent Publication Nos. 2002/0034014, 2004/0099633, 2004/0146807, 2004/0147059 and 2004/0156112 describe state of the art microlens structures and methods of manufacture for image arrays. Basically, the typical method for fabricating a microlens structure comprises: first a layer of photoresist is applied, e.g., by spin coating or like application process (e.g., dip coating, chemical vapor deposition, brushing, evaporation and other like deposition techniques), atop a wafer surface. For instance, as shown in FIG. 1A, the wafer surface may comprise a dielectric planarization layer formed over a substrate 40 that includes an array of color filter structures each associated with an active light sensitive device (e.g., photodiode) of a pixel. It is understood that a positive or negative photoresist may be applied with attendant photolithographic processing steps applied; however, for purposes of discussion, it is assumed a positive photoresist is applied. After a soft bake process, a photoresist mask, such as chrome on glass, is applied having a patterned grid comprising a two-dimensional array of translucent squares or rectangle openings corresponding to the pixel microlens structure to be printed. After aligning the mask to the correct location, the mask and wafer are exposed to a controlled dose of UV light to transfer the mask image. In this example, polymer resist surrounding each exposed square (or rectangular) regions generate acid under uV exposure so that these regions dissolve in a subsequent application of a basic developer chemical. Then, after a post-exposure bake process, a developer step is performed (the kinds of developers that can be employed are well known to those skilled in the art and are dependent on whether a positive or negative photoresist is employed) to remove the soluble areas of the photoresist leaving a visible array pattern of square (or rectangular) shaped islands 42 on the wafer surface separated by thin gaps. Then, as shown in FIG. 1B, after a further blanket expose step, the photoresist pattern is subject to a heating and reflow process to convert the raised photoresist islands into semi-spherical convex lenses 45 of circular plan shape linearly aligned in correspondence with the color filter and active photoelectric converting device.
One problem experienced in the manufacture of pixel microlens structures is that of thin film interference that occurs wherever there is a thin, transparent film. Some light reflects off of the top surface of the photoresist and some light transmits through the photoresist (or any other film for that matter) and reflects off the surface below it. This light then reflects back up, hits the top surface again and either transmits through or reflects back down and interferes constructively or destructively with the light that initially went through. Whether it is destructive or constructive interference depends on whether the light is calumniated (in phase) and, further depends upon the thickness of the layer. If the waves are out of phase they destructively interfere, if they are in phase they constructively interfere. A consequence of either constructive or destructive interference is that it changes the effective dose that is being used during the above-mentioned UV light exposure steps. As shown in FIG. 2A, this is especially apparent at the edges or corner 70 of the metal (MZ) (e.g., aluminum) frame 80 in which a resist layer 64 of thickness “D” used in forming the lens array is formed. As shown in FIG. 2A, when a lens resist is coated (stepped) over the metal frame in the corner of the array, the resist is thicker, as shown as thickness D′, and this changes the thin film interference giving a different dose. The different dose results in the formation of a grossly different lens shape. That is, due to the topography shown as increased thickness D′ at the edge/corner of the array near the metal frame 80, there results little or no process window at the edge of the array (near the frame), and, as shown in FIG. 2B, the formed lenses 95 melt together causing lens variability in size and shape. This is in contrast to the more uniform, correctly-shaped lenses 97 formed further away from the array corners (MZ frame edge) where topographic effects are less an issue.
While it is the case that increased dose and focus centering and dropping the resist thickness assist in countering the small process window phenomenon, the problem is still exacerbated at the array edge/corners, i.e., the MZ fill makes for topography at edge of the frame.
In the manufacture of a pixel sensor array by implementing a process intended to provide touching microlens structures, i.e., a microlens structure having substantially no space between the microlens structure of adjacent pixels to thus maximize light collection, it would be highly desirable to fix the microlens process window to correct for the thin film interference problem exhibited at the corner of the lens array caused by the MZ frame edge topography and not just overexpose to improve the process window but make the gaps between the lens grow.